1. Field of the Invention
The subject invention generally pertains to electronic power conversion circuits, and, more specifically, to high frequency, switched mode electronic power converters. The subject matter relates to new non-isolated power conversion networks which provide high efficiency power conversion under special conditions where more conventional power conversion networks do not work well.
2. Description of Related Art
The solution to the problem of converting power where the input to output or output to input voltage ratio is either very small or very large has traditionally relied upon tapped inductor power converters. An example of a circuit that achieves improved efficiency over the known alternatives for large input to output step down ratios is illustrated in FIG. 1. FIG. 1 is a tapped inductor buck converter. Compared to a simple buck converter the component stress factors for the FIG. 1 circuit are superior to the simple buck converter. M1 in FIG. 1 operates at higher duty cycle and lower current than the high side switch of the simple buck converter and M2 operates at lower duty cycle and higher current than the low side switch of the simple buck converter. The voltage stress of M1 is increased over the voltage stress of the high side switch in the simple buck converter and the voltage stress of M2 is reduced by comparison to the low side synchronous rectifier of the simple buck converter. In the FIG. 1 circuit the component stresses of M1 and M2 are evenly shared, whereas in the simple buck converter the conduction losses are almost all in the low side switch since the duty cycle for the simple buck converter is very small. Also the FIG. 1 circuit has advantages over the simple buck converter in that the switching losses for the M1 switch are reduced in the FIG. 1 circuit, because the current in the M1 switch is reduced. The switch M1 in FIG. 1 experiences an increased voltage stress compared to the high side main switch in a simple buck converter, which offsets some of the advantages of the tapped inductor configuration, yet there is still a net benefit to the tapped inductor approach. The voltage applied to M1 during its off state is equal to the input voltage plus the L1 winding voltage. There is a buck derived converter that is useful for high step down ratios wherein the switch voltage stress never exceeds the input voltage, which is illustrated in FIG. 2. The FIG. 2 circuit is not a tapped inductor circuit and does not provide power to the load during the on time of the main switch M1, but it does provide isolation. What is needed is a tapped inductor buck converter without a voltage stress penalty for the main switch.
A circuit that has grown in popularity in recent years, especially for battery powered circuits is the single ended primary inductor converter (SEPIC), and, particularly, the four switch, dual modulator SEPIC. FIG. 19 illustrates how the four switch dual modulator SEPIC is synthesized from buck and boost converters connected in cascade. The four transistor dual modulator SEPIC uses a single inductor and can convert an input voltage to an output voltage that is either higher than or lower than the input voltage. In many cases the output voltage is near to the input voltage so that the step up or step down ratio is very small. If the output voltage is always less than the input voltage, then a tapped inductor buck converter would be the best solution. If the output voltage was always greater than the input voltage a tapped inductor boost converter would be the best solution. What is needed is a tapped inductor SEPIC converter for the condition where the step up or step down ratio is small, but which can operate as either a step up or step down converter depending on whether the line voltage is less than or greater than the load voltage, respectively.
Both the SEPIC and the tapped inductor buck converter employ a high side switch. A reliable method for driving the high side switch employs a gate drive transformer. As time passes semiconductor solutions are becoming ever cheaper and are smaller than gate drive transformers. Level shifting transistor circuits work well at low voltages, but at high voltages the level shifting circuits are relatively slow and lossy. One semiconductor solution to high side drive is illustrated in FIG. 29. The FIG. 29 circuit achieves an improvement over its prior art by providing positive current feedback around the Schmitt buffer, which rejects changes in the output state of the Schmitt buffer due to small variations in the floating reference voltage (FRV). The positive current feedback achieves an enormous improvement in active clamp circuits where the source voltage of the high side switch varies during the on time of the high side switch. The FIG. 29 circuit is not immune from large fast changes in the floating reference voltage that can cause changes of state of the Schmitt buffer that are not commanded. For example, when the high side switch is commanded off, usually this would coincide with a command to turn on the low side switch, but, if there is a subsequent command to turn off or keep off the low side switch during the transition, then the floating reference voltage of the high side switch will swing low and then swing back high again. When the floating reference swings high, the input to the Schmitt trigger is pulled low, which results in turning the high side switch on again, which will likely not be the desired result. In order to turn off and keep off the high side switch, an OR gate and opto-coupler can be added to independently disable the high side switch, as illustrated in FIG. 30, but this circuit too has some drawbacks. Opto-couplers are relatively slow and the faster opto-couplers draw 10 milliamperes or more of current. They also have physical characteristics that change over time more than other types of semiconductors. They are often disallowed for military, space, and other high reliability applications, because of physical changes with aging. What is needed is a more reliable and faster semiconductor gate drive solution for switches with floating reference voltages, such as the high side switches in the new tapped inductor circuit topologies revealed in the subject application.